MEMS microphone with low pressure region between diaphragm and counter electrode

ABSTRACT

A MEMS microphone includes a first diaphragm element, a counter electrode element, and a low pressure region between the first diaphragm element and the counter electrode element. The low pressure region has a pressure less than an ambient pressure.

TECHNICAL FIELD

Embodiments relate to a microelectromechanical system (MEMS) microphone.Some embodiments relate to a method for manufacturing a MEMS microphone.Some embodiments relate to a MEMS sound transducer. Some embodimentsrelate to a (near-) vacuum microphone and/or a (near-) vacuum speaker.

BACKGROUND

When designing transducers such as pressure sensors, accelerationsensors, microphones, or loudspeakers, it may be typically desirable toachieve a high signal-to-noise ratio (SNR). The continuousminiaturization of transducers may pose new challenges with respect tothe desired high signal-to-noise ratio. Microphones and to some extentalso loudspeakers that may be used in, for example, mobile phones andsimilar devices may nowadays be implemented as silicon microphones ormicroelectromechanical systems. In order to be competitive and providethe expected performance, silicon microphones may need high SNR.However, taking the condenser microphone as an example, the SNR may betypically limited by condenser microphone construction.

The issue of the limited SNR that can be achieved with current designsof condenser microphones, especially when implemented as a MEMS, can beexplained as follows. A condenser microphone may typically comprise adiaphragm and a backplate that may serve as a counter electrode. Thesound may need to pass through the backplate and as a consequence, thebackplate may be typically perforated. Note that the backplate may needto be perforated even in those designs in which the backplate may bearranged behind the diaphragm (i.e., at the side of the diaphragm facingaway from the direction of arrival of the sound), because duringoperation the diaphragm may push some of the air in the volume betweenthe diaphragm and the backplate through the perforated backplate to abackside cavity. Without the backside cavity and the perforation in thebackplate, the volume between the diaphragm and the backplate might actlike a very stiff spring and hence might prevent the diaphragm fromsignificantly vibrating in response to the arriving sound.

A different design of capacitive microphones may use a so-called combdrive where the diaphragm and the counter electrode have a plurality ofinterdigitated comb fingers at a lateral circumference of the diaphragm.These comb sensor microphones may have reduced noise due to the missingbackplate. Still there may be a fluidic element of noise in between theinterdigitated comb fingers.

SUMMARY OF THE INVENTION

A MEMS microphone may be provided. The MEMS microphone may comprise afirst diaphragm element, a counter electrode element and a low-pressureregion between the first diaphragm element and the counter electrodeelement. The low-pressure region may have a pressure less than anambient pressure.

A method for manufacturing a MEMS microphone may be provided. The methodmay comprise creating a low pressure region between a first diaphragmelement and a counter electrode element. The method may further comprisedurably preventing the entry of matter into the low-pressure region todurably maintain, on average, a specified low-pressure within thelow-pressure region.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are described herein makingreference to the appended drawings.

FIG. 1 shows a schematic cross-section of a MEMS microphone comprising asingle diaphragm element and a low-pressure region;

FIG. 2 shows a schematic cross-section of a MEMS microphone, MEMSloudspeaker, or MEMS sound transducer comprising a first diaphragmelement and a second diaphragm element enclosing a low-pressure region;

FIGS. 3A and 3B show schematic cross-sections of the MEMS microphone ofFIG. 2 during operation while being exposed to a sound;

FIG. 4 shows the schematic cross section of FIG. 2 and ad-ditionally aschematic circuit diagram illustrating a power supply and sensingcircuit for the MEMS microphone;

FIGS. 5A and 5B show schematic cross sections of a MEMS microphone at afirst cross-section position;

FIGS. 6A and 6B show schematic cross-sections of the same MEMSmicrophone at a different cross-section position;

FIGS. 7A and 7B show schematic cross sections of the same MEMSmicrophone at a further cross-section position;

FIGS. 8A and 8B show schematic cross sections of the same MEMSmicrophone at yet a further cross-section position;

FIGS. 9A and 9B show schematic cross-sections of the same MEMSmicrophone at a further cross-section position;

FIG. 10 shows a schematic perspective, partial cross-sectional view of aMEMS microphone;

FIG. 11 shows a similar schematic, perspective, partial cross-sectionalview as FIG. 10 to better illustrate some de-tails of the MEMSmicrophone;

FIG. 12 shows a schematic cross-section of a MEMS microphone and theeffect of the atmospheric pressure on the first and second diaphragmelements;

FIG. 13 illustrate a dimensioning of a diaphragm segment spanning thearea between two or more pillars;

FIG. 14 schematically illustrates the amount of bending at the center ofthe diaphragm segment in FIG. 13 at atmospheric pressure as a functionof the thickness and the side length of the diaphragm segment;

FIG. 15 shows a schematic cross-section of a MEMS microphone havinganti-sticking bumps;

FIGS. 16A and 16B show schematic cross sections of a MEMS microphonehaving a laterally segmented counter electrode;

FIGS. 17A and 17B show schematic cross sections of a MEMS microphonecomprising relatively soft diaphragm elements acting as hinges orsuspensions for the first and second diaphragm elements;

FIG. 17C shows a schematic perspective cutaway view of a portion of theMEMS microphone in FIGS. 17A and 17B;

FIG. 18A shows a schematic horizontal cross-section of a MEMS microphonecomprising an X-shaped counter electrode;

FIGS. 18B and 18C show schematic cross sections of the MEMS microphonefrom FIG. 18A;

FIG. 19 shows a schematic cross-section of a MEMS microphone comprisinga single counter electrode and first and second diaphragm elements thatmay be electrically isolated against each other; and

FIGS. 20A to 20O schematically illustrate a process flow of a method formanufacturing a MEMS microphone.

Equal or equivalent elements or elements with equal or equivalentfunctionality are denoted in the following description by equal orequivalent reference numerals.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In the following description, a plurality of details are set forth toprovide a more thorough explanation of embodiments of the presentinvention. However, it will be apparent to those skilled in the art thatembodiments of the present invention may be practiced without thesespecific details. In other instances, well-known structures and devicesare shown in block diagram form rather than in detail in order to avoidobscuring embodiments of the present invention. In addition, features ofthe different embodiments described hereinafter may be combined witheach other, unless specifically noted otherwise.

Standard condenser microphones may use a parallel plate capacitance withchange of gap distance by a membrane displacement. This may imply noiseof air moving through the perforations. When studying the issue of SNRin today's microphones, the perforated backplate may be identified asone of the major noise contributors. One possible solution could be toremove the perforated backplate, but that might need a new sensorconcept. Experiments and simulations performed by the inventors haverevealed that the removal of the perforated backplate could, in theory,improve the SNR by as much as 4 to 27 dB (decibel sound pressure level).For a relatively large microphone having an active volume of 40 mm3, theSNR may be approximately 71 dB(A) with the perforated backplate beingpresent. After the removal of the perforated backplate, the SNR may haveincreased to 98 dB(A). For a relatively small microphone having anactive volume of 2.3 mm3, the improvement may be not as significant, butstill 4 dB from 69 dB(A) with the perforated backplate present to 73dB(A) after removal of the perforated backplate.

Noise in acoustical systems may come from the viscous flow of air in themicrostructures and may cause damping and dissipative losses. For acapacitive microphone concept some aspects described herein may teachhow to encapsulate the static reference electrode (counter electrode)under vacuum or under a low-pressure atmosphere inside the movablemembrane or diaphragm. Further aspects disclosed herein may teach howand under which conditions a low-pressure region can be provided betweena single diaphragm element and the counter electrode element.

FIG. 1 schematically illustrates a concept for a MEMS microphone inwhich a low-pressure region 132 may be provided between a diaphragmelement 112 and a counter electrode element 122. FIG. 1 schematicallyshows a possible embodiment as an example. For reasons of consistencywith subsequent parts of the description, the diaphragm element 112 mayalso be referred to as “first diaphragm element.” The diaphragm element112 may be exposed at one of its sides to an ambient pressure andpotentially a sound pressure. This side of the diaphragm element 112 mayalso be regarded as a sound receiving main surface of the diaphragmelement 112. At its other main surface the diaphragm element 112 may beadjacent to the low-pressure region 132. The diaphragm element 112 maybe implemented as a membrane or membrane element. A displacement of thediaphragm element 112 in response to the sound pressure may beschematically illustrated in FIG. 1 by dash-dot-dot lines (note that thedisplacement may be shown somewhat exaggerated for illustrativepurposes).

The low-pressure region 132 may be schematically illustrated in FIG. 1by a dashed line. The low-pressure region 132 has a pressure that may betypically less than an ambient pressure or a standard atmosphericpressure. The low-pressure region 132 may be adjacent and typically indirect contact with the diaphragm element 112 and also with the counterelectrode element 122.

The diaphragm element 112 may be biased by a pressure difference betweenthe ambient pressure and the pressure within the low-pressure region132, which typically may be less than the ambient pressure. Accordingly,the diaphragm element 112 may assume a corresponding rest position orconfiguration when no sound arrives at the diaphragm element 112. Thelower pressure may result in lower damping according to the density ofthe fluid inside the low pressure or vacuum region. At the same time themembrane withstanding the normal pressure and sensing the sound may notneed any back volume since there might be less or no force transferredto the second electrode via a fluidic coupling. To give some numbers asan example, the membrane might have to withstand an absolute pressure ofup to about 100 kPa. The sound pressure to be sensed may be, forexample, in a range of up to about 1 mPa or up to 10 mPa.

According to at least one embodiment, the pressure in the low pressureregion may be substantially a vacuum or a near-vacuum. In other examplesof implementation the pressure in the low pressure region may be lessthan about 50% of the ambient pressure or the standard atmosphericpressure. It may also be possible that the pressure in the low pressureregion may be less than about 45%, 40%, 35%, 30%, 25%, or 20% of theambient pressure or the standard atmospheric pressure (standardatmospheric pressure may be typically 101.325 KPa or 1013.25 millibars).The pressure in the low pressure region may also be expressed as anabsolute pressure, for example less than 50 KPa, less than 40 KPa, lessthan 30 KPa, or less than 25 KPa. In any event, the pressure in the lowpressure region may be typically selected such that it may be lower thanthe typical range of the atmospheric pressure for weather conditionsthat should be reasonably expected and for those altitudes with respectto sea level at which the MEMS microphone may be intended to be usable(e.g., up 9000 meters above sea level).

The first diaphragm element may have a diaphragm compliance of at leastabout 1 nm/Pa. According to alternative implementations the diaphragmcompliance may be at least about 2 nm/Pa, at least about 3 nm/Pa, atleast about 4 nm/Pa, or at least about 5 nm/Pa. The diaphragm compliancemay typically be understood as the inverse of the diaphragm's stiffness.However as used herein, the diaphragm compliance may be normalized tothe size of the diaphragm and may express a maximum deflection of thediaphragm when being charged with a specific sound pressure, here 1Pascal (Pa). The reference sound pressure in air that may be commonlyused may be Pref=20 μPa(rms), which approximately corresponds to thethreshold of human hearing. With this reference sound pressure, a soundpressure level (SPL) of 94 dB may result in a sound pressure of 1 Pa(for comparison, a jack hammer at 1 meter may have a sound pressurelevel of approximately 100 dB).

FIG. 2 shows a schematic cross-section through a MEMS microphone thatmay further comprise a second diaphragm element 214 disposed on anopposite side of the counter electrode element 222 than the firstdiaphragm element 212. FIG. 2 shows a further possible embodiment. TheMEMS microphone may comprise a plurality of pillars or struts 272extending between the first diaphragm elements 212 and the seconddiaphragm element 214. The pillars 272 typically do not contact or touchthe counter electrode element 222, but rather may pass through thecounter electrode element 222 via openings or holes 227 in the counterelectrode element 222. In the implementation example schematicallyillustrated in FIG. 2, the pillars 272 may be integrally formed with thefirst and second diaphragm elements 212, 214. Hence, the first diaphragmelement 212, the second diaphragm element 214, and the pillars 272 mayform an integral structure of the same material, for examplepolycrystalline silicon. Nevertheless, this does not mean that the firstdiaphragm element 212, the second diaphragm element 214, and the pillars272 need to be formed concurrently during manufacture of the MEMSmicrophone. Rather, it may be possible that the second diaphragm element214 may be formed first on a surface of a substrate 202 (or on a surfaceof an auxiliary layer such as an etch stop layer) during a firstdeposition process. Subsequently, the pillars 272 and eventually alsothe first diaphragm element 212 may be formed during a second depositionprocess and possibly during a third deposition process. In alternativeimplementation examples to be described below, the pillars 272 may bemade of a different material than the first and second diaphragmelements 212, 214. The first diaphragm element 212 may have a mainsurface that may face the direction of arrival of a sound (schematicallyillustrated by an arrow in FIG. 2).

In the MEMS microphone schematically illustrated in cross-section viewin FIG. 2 as an example, a second counter electrode element 224 may beprovided in addition to the first counter electrode element 222. Thesecond counter electrode element 224 may be spaced apart from the firstcounter electrode element 222. A counter electrode isolating layer 252electrically may isolate the first counter electrode element 222 and thesecond counter electrode element 224 against each other. In the exampleof a MEMS microphone schematically illustrated in FIG. 2, the firstcounter electrode element 222, the second counter electrode element 224,and the counter electrode isolating layer 252 may form a counterelectrode arrangement or counter electrode structure that may besupported at its periphery or circumference by a support structure. Notethat although the three central portions of the counter electrodearrangement depicted in FIG. 2 appear to be “floating” within the lowpressure region 232, they may be typically attached to the circumferenceof the counter electrode structure above and/or beneath the drawingplane of FIG. 2, as indicated by the dashed lines.

In the example schematically illustrated in FIG. 2, the supportstructure may have a stacked configuration and peripheral portions ofthe first diaphragm element 212, the second diaphragm element 214, andthe counter electrode arrangement 222, 224, 252 may be in planar contactwith the support structure at one or two of their main surfaces. Thesupport structure itself may be arranged at a main surface of thesubstrate 202. On this main surface of the substrate 202 the variouslayers may be arranged on top of each other in the following order, forexample: second diaphragm element 214, second diaphragm isolation 244,second counter electrode element 224, counter electrode isolation 252,first counter electrode element 222, first diaphragm isolation 242, andfirst diaphragm element 212. A backside cavity 298 may be formed in thesubstrate 202 in order to allow the second diaphragm element 214 tooscillate in response to a sound wave.

When studying the pressure situation of the structure, it can beobserved that the diaphragm structure which comprises the firstdiaphragm element 212, the pillars 272, and the second diaphragm element214 may have to be stiff enough to withstand the 1 bar overpressure ofthe outer atmosphere against the vacuum cavity or low-pressure cavity.In particular, the pillars 272 may be regarded as vertical ridgesreaching through holes 227 of the counter electrode arrangement (alsocalled “stator”) in order to stabilize the structure. The diaphragmarrangement 212, 214 may be tightly sealed.

FIG. 2 shows the MEMS microphone at its rest position, e.g. when nosound wave arrives at the diaphragm elements 212, 214 which would causethe diaphragm elements 212, 214 to be deflected. At the side of thefirst diaphragm element 212 at which the sound may arrive, the totalpressure may be expressed as p(t)=normal pressure+psound(t). Within thebackside cavity 298, only the normal atmospheric pressure may bepresent, i.e. p0=normal pressure. Within the low-pressure region 232,the pressure may be relatively low, e.g. pgap˜0 or pgap<50% ambientpressure.

FIGS. 3A and 3B show schematic cross sections through a possible MEMSmicrophone when the same may be exposed to sound as possible examplesand/or embodiments. FIG. 3A shows the situation in which the diaphragmarrangement 212, 214, 272 may be pushed down due to a relativeoverpressure caused by the sound at the upper side adjacent to the firstdiaphragm element 212 compared to the reference pressure within thebackside cavity 298, i.e.,p(t)=normal pressure+|psound|.

In FIG. 3B the pressure at the sound receiving side may be lower thanthe pressure within the backside cavity 298 so that the diaphragmarrangement 212, 214, 272 may be deflected upwards. Accordingly, thediaphragm structure or membrane structure moves up and down with respectto the counter electrode structure 222, 224, 252 (stator) under sound.The underpressure in FIG. 3B can be expressed asp(t)=normal pressure−|psound|.

FIG. 4 schematically illustrates an example of how the MEMS microphonemay be electrically connected to a power supply circuit and anamplifier. FIG. 4 shows an example of a possible connection. Otherarrangements may be possible, too. In FIG. 4, the first and seconddiaphragm elements 212, 214 may be grounded by a diaphragm connection412 to an electric ground potential or reference potential. The firstcounter electrode element 222 may be electrically connected by a firstcounter electrode connection 422 to a first power supply circuit andalso to a first input of an amplifier 401. The first power supplycircuit comprises a voltage source 402 and a resistor 406. The resistor406 may have a very high resistance of several Giga Ohms or even as highas 1 Tera Ohm. The amplifier 401 may be a differential amplifier. Thesecond counter electrode element 224 may be connected by a secondcounter electrode connection 424 to a second power supply circuit and asecond input of the amplifier 401. The second power supply circuitcomprises a second voltage source 404 and a second resistor 408 thattypically has about the same resistance as the resistor 406. The firstand second power supply circuit electrically biases the first and secondcounter electrode elements 222 and 224, respectively, against theelectric reference potential (ground potential). When the diaphragmstructure may be deflected in response to an arriving sound, theelectric potentials at the first and second counter electrode elements222, 224 may vary in opposite directions due to the varying capacitancesbetween the diaphragm structure and the first and second counterelectrode elements, respectively. This is schematically illustrated inFIG. 4 by a first waveform 432 and a second waveform 434 which may befed into the first and second input, respectively, of the amplifier 401.The amplifier 401 may generate an amplified output signal 430 based onthe input signals 432 and 434, in particular a difference of the inputsignals 432, 434. The amplified output signal 430 may then be suppliedto further components for subsequent signal processing, for exampleanalog-to-digital conversion, filtering, etc.

A possible implementation of a MEMS microphone having a low-pressureregion between two diaphragm elements and a counter electrode within thelow-pressure region will now be described with respect to FIGS. 5A to10. FIGS. 5A to 10 show possible embodiments and/or examples of possibleimplementations. The FIGS. 5A, 6A, 7A, 8A, and 9A may be substantiallyidentical and indicate a location of a corresponding horizontal crosssection shown in FIGS. 5B, 6B, 7B, 8B, and 9B, respectively. The exampleschematically illustrated in FIGS. 5A to 10 may relate to a lateraldesign including a ventilation hole 515 for static pressure equalizationbetween the ambient atmosphere and the backside cavity 298.

FIG. 6A indicates that the next horizontal cross section, which isschematically illustrated in FIG. 6B, may be performed according to asection plane passing through the second diaphragm element 214. Theventilation hole 515 may have a square cross-section at this position.

FIG. 7A shows another schematic cross-section of the MEMS microphone andFIG. 7B shows the corresponding schematic horizontal cross section thatmay have been performed at a height of the second diaphragm isolation244. In the depicted example of a MEMS microphone, the second diaphragmisolation 244 may not only provide an electrical isolation between thesecond diaphragm element 214 and the second counter electrode element224, but may also serves as a support for the second counter electrodeelement 224 and other structures that may be arranged on top of thesecond counter electrode element 224. Hence, the second diaphragmisolation 244 can be regarded as a part of a support structure, as well.The second diaphragm isolation 244 may also laterally confine or limitthe low-pressure region 232. The pillars 272 can also be seen in FIGS.7A and 7B. In a similar manner as the pillars 272, a channel 715 may beformed by four sidewalls that extend between the first diaphragm element212 and the second diaphragm element 214. The channel 715 may have asquare cross-section in the depicted example, but may have othercross-sectional shapes, as well. The channel 715 may seal thelow-pressure region 232 against the ventilation hole 515.

In FIG. 7B it can be seen that each of the pillars 272 may have anelongate cross-section, in particular a rectangular cross-section.However, other cross-sectional shapes may be possible, as well. Hence,each of the pillars 272 may be significantly wider than thick, forexample between three times and six times more wide than thick. Thewidth of a pillar may be schematically indicated in FIG. 7B as “w” andthe thickness of a pillar 272 may be schematically indicated in FIG. 7Bby “t”. A first subset of the pillars 272 may be oriented such thattheir cross-sectional width w extends along a first direction. A secondsubset of the pillars 272 may be oriented differently such that theircross-sectional width w extends in a second direction that may be notparallel to the first direction. In the example schematicallyillustrated in FIG. 7B, the second direction of cross-sectionalorientation of the second subset of pillars 272 may be orthogonal to thefirst direction which describes the cross-sectional orientation of thepillars 272 within the first subset of pillars. In alternativeembodiments, the plurality of pillars 272 could be subdivided in threeor even more subsets of pillars each having different directions ofcross-sectional orientation. The pillars 272 have differentcross-sectional orientations in order achieve a substantially isotropicstiffness of the entire diaphragm arrangement against the overpressureexerted by the atmosphere onto the first and second diaphragm elements212, 214. Furthermore, the at least some of the pillars 272 may bespaced apart from each other to leave sufficient space for the counterelectrode arrangement 222, 224, 152, as will be seen in FIG. 8B.

FIG. 8B shows a horizontal cross-section at the height of the counterelectrode isolation 252. The geometry of the counter electrode isolation252 may be in the depicted example also representative of the geometriesof the first and second counter electrode elements 222 and 224, and thusfor the entire counter electrode arrangement comprising the three layersof first counter electrode 222, counter electrode isolation 252, andsecond counter electrode element 224. The counter electrode isolation252 may comprise holes 227. The pillars 272 may pass through the holes227 without contacting the rims of the holes 227, i.e. with sufficientclearance. Thus, the diaphragm arrangement may move up and down withrespect to the counter electrode arrangement when the diaphragmarrangement may be deflected up or down, which may happen mainly withinits central portion when the diaphragm arrangement is exposed to a soundwave. Furthermore, the holes 227 within the counter electrodearrangement may prevent that the pillars 272 get into electrical contactwith the first counter electrode 222 and/or the second counter electrode224, which would cause a short circuit between the diaphragm arrangementand the counter electrode arrangement.

In order to provide some additional mechanical stability to thediaphragm arrangement, the sidewalls of the channel 715 in FIG. 8B maybe thicker than in the horizontal cross section of FIG. 7B. Theventilation hole 515 may have a circular cross section at the positionshown in FIG. 8B.

FIG. 9B shows a similar horizontal cross-section as FIG. 8B with thedifference of the cross-section being performed at the height of thefirst counter electrode element 222.

FIG. 10 shows a schematic, perspective cross-section view of the MEMSmicrophone. FIG. 11 shows a similar schematic, perspective cross-sectionview in which the relation of the counter electrode arrangement and thepillars 272 may be shown in further detail. In particular, FIG. 11 mayshow how one of the pillars 272 may pass through one of the holes 227that may be formed in the counter electrode arrangement.

FIG. 12 shows a schematic cross-section of a MEMS microphone in which abending of diaphragm sections may be schematically illustrated. Due tothe vacuum or low-pressure between the first and second diaphragmelements 212, 214, the suspended diaphragm parts may be loaded withambient pressure resulting in a bending. Due to the pillars 272 that maybe typically regularly arranged between the first and second diaphragmelements 212, 214, the bending can be reduced to a relatively smallamount.

FIG. 13 schematically illustrates one suspended diaphragm part (membranepart). The lateral dimension “l” of the suspended diaphragm part, itsthickness t_(diaphragm), and its intrinsic stress may define the amountof bending. As an example, FIG. 14 graphically illustrates the resultsof calculations for the bending under 1 bar pressure (atmosphericpressure) of a small square segment of a stress-free polysilicondiaphragm for different thicknesses and side lengths. For typicaldimensions (side length=20 μm, thickness=0.5 μm) the bending may beabout 140 nm and acceptable for an air gap of typically 2 μm. Tensionalstress in the diaphragm layer may additionally reduce the bending.

FIG. 15 shows a schematic cross-section of a MEMS microphone accordingto a possible embodiment that may have a low pressure region 232 betweenthe first and second diaphragm elements 212, 214. According to theimplementation example schematically illustrated in FIG. 15, the firstdiaphragm element 212 may comprise anti-sticking bumps 1512 that may bearranged at the surface of the first diaphragm element 212 that may facethe low-pressure region 232. The anti-sticking bumps 1512 may reduce arisk of the first diaphragm element 212 getting stuck at the firstcounter electrode element 222 due to an adhesive force. In a similarmanner, the second counter electrode element 224 may comprise a secondplurality of anti-sticking bumps 1524 facing the second diaphragmelement 214. The anti-sticking bumps 1512 may be integrated with thefirst diaphragm element 212. The anti-sticking bumps 1524 may beintegrated as one part with the second counter electrode element 224.

FIG. 16A shows a schematic cross-section of a MEMS microphone having alateral segmentation of the counter electrodes. FIG. 16B shows aschematic horizontal section of the same MEMS microphone. In thisembodiment, the first counter electrode element 222 does not extend intothe support structure anymore except for a small contact strip forelectrically contacting the first counter electrode element 222 withexternal circuitry such as power supply and read-out circuits. The firstcounter electrode element 222 may be laterally delimited by a gap 1623that may electrically isolate the first counter electrode element 222from a region of surrounding counter electrode material 1622. The firstcounter electrode element 222 may be limited to a center region of theMEMS microphone. The first diaphragm element 212 and the seconddiaphragm element 214 may undergo larger deflection due to an excitationby a sound wave in the center region than in a margin region. In themargin region, i.e. within the support structure and in the vicinity ofthe support structure, the first and second diaphragm elements 212, 214may typically not significantly move in response to the sound wave.Therefore, the margin region might not contribute to a variation of thecapacitances. The lateral segmentation of the first and second counterelectrode elements 222, 224 may typically result in a larger percentagevariation of the capacitance in response to a sound wave andconsequently to a higher sensitivity of the MEMS microphone. The gap1623 may be filled with the material of first diaphragm isolation 242when passing through the support region in order to seal thelow-pressure region 232 against the exterior ambient atmosphere. Thesame can be done with the gap between the second counter electrodeelement 224 and the corresponding margin material 1624 in that thesecond diaphragm isolation 244 may be used to fill the gap between theelements 224 and 1624. In the alternative, the gap 1623 and the gaparound the second counter electrode element 224 may be filled with orreplaced by dedicated isolating material.

FIG. 17A shows a schematic cross section of a MEMS microphone as anexample of how softer diaphragms or membranes may be introduced and maystill be rigid against the low-pressure within the low-pressure region,i.e. between the first and second diaphragm elements 212, 214. FIG. 17Bshows a corresponding horizontal section. The MEMS microphone maycomprise a hinge element or third diaphragm element 1716. The hingeelement or third diaphragm element 1716 may be coupled between the firstdiaphragm element 212 and a support structure 1706. The hingeelement/third diaphragm element 1716 may have a stiffness which may beless than the stiffness of the first diaphragm element 212 and/or lessthan the stiffness of the second diaphragm element 214. The thirddiaphragm element 1716 may comprise a wall element 1717 configured tolaterally confine the low-pressure region 232. The wall element 1717 maybe coupled to the support structure 1706 so that the support structure1706 may participate in confining the low-pressure region 232. The MEMSmicrophone schematically illustrated in FIGS. 17A and 17B may comprisefour hinge elements/third diaphragm elements 1716. The first counterelectrode element 222 may be coupled to the support structure 1706independently from the hinge element 1716. This may be achieved byproviding at least one gap in the hinge element 1716 through which thefirst counter electrode element 222 may extend from the low-pressureregion 232 to the support structure 1706 in FIG. 17B. This may beschematically illustrated for the counter electrode isolation 252. Thestructure of the first counter electrode element 222 and the secondcounter electrode element 224 may be substantially similar to thestructure of the counter electrode isolation 252. In the configurationshown in FIGS. 17A and 17B, there may be four gaps provided between thefour hinge elements 1716, the four gaps being provided, for example, inthe four corners of a square formed by the four hinge elements 1716.

FIG. 17C shows a schematic perspective cutaway view of the first andsecond diaphragm elements 212, 214 and two of the hinge elements 1716.For the sake of clarity, the counter electrode elements 222, 224 and thecounter electrode isolation 252 have been omitted from illustration inFIG. 17C. It can be seen that each of the hinge elements 1716 may form astructure that can be described as a “double-trough” with the twotroughs being arranged bottom-to-bottom to each other. In the corner,the two hinge elements 1716 do not necessarily meet each other and mayleave the gap between the hinge elements 1716 which may allow thecounter electrode structure to be mechanically and electrically coupledto the support structure independently from the diaphragm structure. Thehinge elements 1716 may have an H-shaped cross section in thisimplementation example. In alternative implementations, the hingeelement(s) 1716 could have, for example, a U-shaped cross section oranother cross section, where for example the second diaphragm element214 may be continuous to form the lower bar of the “U” and the firstdiaphragm element 212 may be interrupted by the wall element 1717. Thedashed lines in FIG. 17C may schematically indicate some of the innercontours of the low pressure region 232.

FIGS. 18A to 18C schematically illustrate a further possibleimplementation of a MEMS microphones wherein the counter electrodeelements may have approximately an X-shaped configuration. FIG. 18A mayshow a schematic top view of the first counter electrode element 1822and of the hinge elements or third diaphragm element 1816. For the sakeof clarity, some elements may have been omitted from illustration, forexample the wall elements 1717 in FIGS. 17A to 17C. It can be seen thatthe first counter electrode element 222 may be suspended at the supportstructure 1706 by four arms that may extend in an X-shaped manner from acentral portion of the first counter electrode element 222. As analternative implementation, the first counter electrode element 1822could be supported at the support structure by only one arm, two arms,three arms, or any other number of arms.

FIG. 18B shows a schematic cross-section through the MEMS microphone ofFIG. 18A. FIG. 18C shows a corresponding horizontal section through theMEMS microphone. As can be seen in FIG. 18C, the cross-section of FIG.18B may be done at an angled section plane so that a left portion inFIG. 18B shows a cross-section through the hinge element 1816 and aright portion of FIG. 18B shows a schematic cross-section through thecounter electrode isolation 1852. The hinge element or third diaphragmelement 1816 may comprise corrugation lines 1818 that promote a bendingof the hinge element 1816 in this area. The bending of each hingeelement 1816 may be described as a rotation about an axis that extendsparallel to an elongate extension of the corrugation lines 1818. Thewall element 1817 of the hinge element 1816 may participate in confiningthe low-pressure region 232 against the ambient atmosphere. To this end,the wall element 1817 may be coupled to the support structure 1706. Inthe example shown in FIGS. 18A to 18C, the wall element 1817 maycomprise a first wall portion that may start at the support structure1706 at an angle, a second wall portion that may extend substantiallyparallel to the support structure 1706, and a third wall portion thatmay merge with the support structure 1706 at an angle. In this manner,the wall element 1817 may form three sides of a trapezoid that surroundsthe remainder of the hinge element 1816, in particular the portioncomprising the corrugation lines 1818. A fourth side of the trapezoidmay be formed by the support structure. Further vent holes 1815 may beformed within one or more of the hinge element(s) 1816. The ventilationholes 1815 may be configured to facilitate a static pressureequalization between the ambient pressure and the backside cavity 298.As explained above, there may also a be further vent hole 515 in acentral pillar 715.

FIG. 19 shows a further example of a possible implementation of a MEMSmicrophone where the stator, i.e. the counter electrode arrangement, maybe realized as a single electrode and the movable diaphragm structurecomprises two electrodes that may be electrically isolated against eachother. The MEMS microphone may comprise a first diaphragm element 1912and a second diaphragm element 1914. The first diaphragm element 1912may be mechanically coupled to the second diaphragm element 1914 via aplurality of electrically isolating pillars 1972. The counter electrodearrangement may comprise a single counter electrode element 1922 ofelectrically conducting material. It may be also possible to provide twocounter electrodes that may be electrically isolated against each other,and additionally two diaphragms, that may be also electrically isolatedagainst each other, i.e., four different electrodes for the MEMSmicrophone.

FIGS. 20A to 20O show schematic cross sections through a portion of awafer during various stages or steps of a possible example for amanufacturing process of a MEMS microphone as described above. Anydimensions, thickness values of the various layers, material selections,etc. are examples, only, and may therefore by changed.

FIG. 20A shows the substrate 202 which may be a silicon wafer in whichsilicon may be arranged in mono-crystalline structure. A lower etch stoplayer 203 may have been deposited at an upper main surface of thesubstrate 202. The lower etch stop layer 203 may ensures a reliable stopof an etching process for forming the cavity 298 which may occur at alater stage of the manufacturing process. The lower etch stop layer 203may be typically made from an oxide, a thermal oxide, or TEOS, forexample. It's thickness may be between 0.1 and 1 μM.

FIG. 20B shows a schematic cross-section of the wafer after a layer forthe second diaphragm element 214 has been deposited on the lower etchstop layer 203. Furthermore, the second diaphragm element 214 may alsobe already structured in FIG. 20B. The material may be dopedpoly-silicone which may be deposited as a doped polysilicon layer aspart of the motor of the MEMS microphone. The layer 214 may be typicallybetween 0.5 and 2 μm thick.

FIG. 20C shows a schematic cross-section through the wafer after a layerof sacrificial oxide 2044 for a lower gap has been deposited on thestructure shown in FIG. 20B. The sacrificial oxide may be substantiallythe same material as the material for the lower etch stop layer 203. Thethickness of the deposited second diaphragm oxide 2044 on top of thesecond diaphragm element 214 may be typically between about 0.5 and 2μm, depending on the desired gap width for the MEMS microphone.

FIG. 20D shows a schematic cross-section after the various layers of amultilayer stator have been deposited on the previously depositedsacrificial oxide 2044. The multilayer stator may comprise in thedepicted example three layers: a layer 2024 for subsequently forming thesecond counter electrode element 224, a layer 2052 of electricallyinsulating material for subsequently forming the counter electrodeisolation 252, and a layer 2022 for subsequently forming the firstcounter electrode element 222. The layers 2024 and 2022 may be of dopedpolysilicon or comprise doped polysilicon. The layer 2052 may comprisesilicon nitride SiN. Other materials may be also possible, for examplemonocrystalline silicon (bulk or silicon-on-insulator, SOI),poly-crystalline silicon, metal (e.g., aluminum or a AlSiCu). Dielectriclayers may comprise an oxide, Si3N4, Si_(x)N_(y)O, poly imide, etc. Thethicknesses of the various layers of the multilayer stator may be, forexample between about 0.1 and 1 μm for the layers of the first andsecond counter electrode elements 2022, 2024 and between about 0.1 and0.5 μm for the layer of the counter electrode isolation 2052.

FIG. 20E shows a schematic cross-section after the multilayer statorcomprising the three layers 2024, 2052, and 2022 may have beenstructured and, in particular, openings 2027 or trenches may have beenformed in the multilayer stator, said openings 2027 possibly extendingto the second diaphragm isolation layer 2044, for example.

The openings 2027 may be then filled by means of a deposition process,for example a TEOS deposition 2042 with a thickness between about 0.5and 5 μm. In case the second diaphragm isolation layer 2044 may be ofthe same material than the deposited material, the two layers may mergeand may form one structure. A schematic cross-section after a TEOSdeposition may be shown in FIG. 20F. Other deposition materials may bepossible, as well.

FIG. 20G shows a schematic cross-section after a mask 2045 may have beendeposited on the second diaphragm isolation layer 2042 and structured.Then, a so-called spacer etch process (pillar etch process) may beperformed, the results of which can be seen in FIG. 20H. In particularthe holes 2027 may have been extended with respect to their depth sothat they now may reach down to the second diaphragm element 214.

In FIG. 20I the mask 2045 may have been removed. The holes 2027 may nowdefine the shape of the future pillars 272. In a subsequent step afurther deposition of doped polysilicon 2012 may be performed whichfills the holes 2027 (FIG. 20J). The thickness of the deposited dopedpolysilicon may be between about 0.5 and 2 μm, for example.

Subsequently, the deposited doped polysilicon 2012 may be structured. Bystructuring the first diaphragm layer 2012, a plurality of small holes2011 may be created in the first diaphragm layer 2012. Each hole mayhave a diameter of, for example, between about 0.1 and 1 μm. The smallholes 2011 may subsequently be used as etch holes and then closed again.FIG. 20J shows a schematic cross-section after the first diaphragm layer2012 has been deposited and structured. Concurrently with the formationof the small etch holes 2011, a lateral segmentation of the firstdiaphragm layer 2012 may be performed by forming a gap 2021 which mayseparate the first diaphragm 212, which may be completed subsequently,from a surrounding portion of the first diaphragm material 2012. Thesurrounding portion of the first diaphragm material may subsequently beused to electrically contact the first counter electrode element 222,the second counter electrode element 224, and/or the second diaphragmelement 214.

FIG. 20K shows a schematic cross-section after the lateral segmentation2021 may have been temporarily covered by means of a mask 2046. Usingthe remaining small holes 2011 that have not been masked, a release etchmay be performed in order to remove the oxide between the seconddiaphragm layer 214 and the first diaphragm layer 2012. The release etchprocess may be time-controlled so that a margin portion of thesacrificial material 2042, 2044 might not be etched away by the etchingagent because the distance of the nearest hole 2011 may be too large forthe etching agent to reach this margin portion during the duration ofthe release etch process. Instead of a time-controlled etching process,other forms for providing an etch stop may be used, as well.

FIG. 20L shows a schematic cross section after the mask 2046 may havebeen removed. In FIG. 20M an etching hole closure may have beenperformed to close the small holes 2011 with a suitable closure material2019, which may be schematically indicated in FIG. 20M and thesubsequent FIGS. 20N and 20O by a thick line. This closure step may beperformed under a low pressure atmosphere or a (near-) vacuum in orderto obtain the low pressure region 232. The etching hole closure maycomprise one or more of the following actions:

coating with a non-conformal deposition of oxide under low pressure, or

deposition of BPSG (borophosphosilicate glass) and later reflow underlow pressure/vacuum, or

lamination of a foil under low pressure/vacuum.

FIG. 20M illustrates the case of deposition of BPSG which also mayresult in the BPSG to cover the inner side walls of the low pressureregion 232.

FIG. 20N shows a schematic cross section of a MEMS microphone duringmanufacture after contact holes may have been etched. A first contact2082 may be formed within a first contact hole and may provide anelectrical connection for the first diaphragm element 212. A secondcontact 2092 may be provided within a second contact hole to provide anelectrical connection for the first counter electrode element 222. Athird contact 2094 may be provided within a third contact hole as anelectrical connection for the second counter electrode element 224. Notethat the lateral segmentation(s) 2021, the first diaphragm isolation242, and the counter electrode isolation 252 provide electricalisolation between the different contacts 2082, 2092, 2094, for example.A contact for the second diaphragm element 214 may be not explicitlyshown in FIG. 20N, but can be formed in an analog manner than thecontacts 2082, 2084, 2092, for example.

FIG. 20O shows the final MEMS microphone in schematic cross sectionafter backside etching of the backside cavity 298, for example by meansof a DRIE/Bosch Process (DRIE: Deep Reactive Ion Etching). The loweretch stop layer 203 may act as an etch stop for the DRIE process and mayhave been removed after the DRIE process by a further dedicated oxideetching process.

Although some aspects have been described in the context of a device, itis clear that these aspects also represent a description of thecorresponding method, where a block or device corresponds to a methodstep or a feature of a method step. Analogously, aspects described inthe context of a method step also represent a description of acorresponding block or item or feature of a corresponding device.

The above described embodiments are merely illustrative for theprinciples of the present invention. It is understood that modificationsand variations of the arrangements and the details described herein willbe apparent to others skilled in the art. It is the intent, therefore,to be limited only by the scope of the impending patent claims and notby the specific details presented by way of description and explanationof the embodiments herein.

Although each claim only refers back to one single claim, the disclosurealso covers any conceivable combination of claims.

What is claimed is:
 1. A MEMS microphone comprising: a first diaphragmelement; a second diaphragm element spaced apart from the firstdiaphragm element; a low pressure region between the first diaphragmelement and the second diaphragm element, the low pressure region havinga pressure less than an ambient pressure; a first counter electrodeelement disposed within the low pressure region; and a second counterelectrode element spaced apart from the first counter electrode elementand disposed within the low pressure region.
 2. The MEMS microphoneaccording to claim 1, wherein the pressure in the low pressure region issubstantially a vacuum.
 3. The MEMS microphone according to claim 1,wherein the pressure in the low pressure region is less than about 50%of the ambient pressure.
 4. The MEMS microphone according to claim 1,wherein the low pressure region is formed by the first diaphragm elementand the second diaphragm element.
 5. The MEMS microphone according toclaim 1, wherein the first diaphragm element is electrically coupled tothe second diaphragm element.
 6. The MEMS microphone according to claim1, wherein the first diaphragm element is electrically isolated from thesecond diaphragm element.
 7. The MEMS microphone according to claim 1,wherein the first counter electrode element is electrically isolatedfrom the second counter electrode element.
 8. The MEMS microphoneaccording to claim 1, wherein the first diaphragm element has adiaphragm compliance of at least about 1 nm/Pa.
 9. The MEMS microphoneaccording to claim 1, wherein the first diaphragm element has adiaphragm compliance of at least about 5 nm/Pa.
 10. The MEMS microphoneaccording to claim 1, wherein the low pressure region is within a sealedcavity.
 11. The MEMS microphone according to claim 1, further comprisinga hinge element coupled between the first diaphragm element and asupport structure.
 12. The MEMS microphone according to claim 11,wherein the hinge element comprises a wall element configured tolaterally confine the low pressure region.
 13. The MEMS microphoneaccording to claim 12, wherein the wall element is coupled to thesupport structure so that the support structure participates inconfining the low pressure region.
 14. The MEMS microphone according toclaim 12, wherein the counter electrode element is coupled to thesupport structure independently from the hinge element through at leastone gap in the hinge element extending from the low pressure region tothe support structure.
 15. A MEMS microphone, comprising: a firstdiaphragm element; a second diaphragm element spaced apart from thefirst diaphragm element; a low pressure region disposed between thefirst diaphragm element and the second diaphragm element, the lowpressure region having a pressure less than an ambient pressure; a firstcounter electrode element disposed within the low pressure region; andone or more pillars coupled between the first diaphragm element and thesecond diaphragm element, wherein the one or more pillars areelectrically conductive.
 16. The MEMS microphone of claim 15, whereinthe pressure in the low pressure region is substantially a vacuum. 17.The MEMS microphone of claim 15, wherein the pressure in the lowpressure region is less than 50% of the ambient pressure.
 18. The MEMSmicrophone of claim 15, wherein the low pressure region is within asealed cavity.
 19. The MEMS microphone of claim 15, further comprises asecond counter electrode element electrically isolated from the firstcounter electrode element and arranged in the low pressure region. 20.The MEMS microphone according to claim 15, wherein at least two pillarsare spaced apart from each between 5 μm and 20 μm.
 21. The MEMSmicrophone of claim 15, further comprising a third diaphragm element,the third diaphragm element having a stiffness which is less than thestiffness of the first diaphragm element or the stiffness of the seconddiaphragm element.
 22. The MEMS microphone of claim 21, wherein thethird diaphragm element is coupled between a support structure and atleast one of the first diaphragm element and the second diaphragmelement.
 23. The MEMS microphone of claim 22, wherein the first counterelectrode element is supported at the support structure independentlyfrom the third diaphragm element.
 24. A MEMS microphone, comprising: afirst diaphragm element; a second diaphragm element spaced apart fromthe first diaphragm element; a low pressure region disposed between thefirst diaphragm element and the second diaphragm element, the lowpressure region having a pressure less than an ambient pressure; a firstcounter electrode element disposed within the low pressure region; and athird diaphragm element, the third diaphragm element having a stiffnesswhich is less than the stiffness of the first diaphragm element or thestiffness of the second diaphragm element.